1. Field of the Invention
The present invention relates to a dimensional control device for cassettes used to transport semiconductor wafers from one processing site to another.
2. Discussion of the Related Art
FIGS. 1A and 1B are a front view and a top partial cross-sectional view of a conventional cassette to transport wafers.
In FIG. 1A, the cassette is shown from the side of an aperture through which the wafers are inserted. The cassette includes a base 12 from which extend, on both sides, two vertical walls 13. Walls 13 include horizontal grooves 15 with trapezoidal section. Each groove 15 of one wall 13 is in the same plane as a groove 15 of the other wall 13, so that both these grooves form a single groove designed to receive a wafer. A wafer 10 is shown in place.
As shown in FIG. 1B, walls 13 include a linear portion 13-1 at the side of the insertion aperture, and a curved portion 13-2 at the rear, which matches the shape of the wafers and forms a stop for them.
The cassettes are used to transport sets of wafers from one processing site to another. At each site, the cassettes are loaded and unloaded one after the other with a robot. Of course, for the robot to be able to suitably handle the wafers, it is necessary that the positions of the grooves 15 vary little with respect to corresponding reference positions.
For this purpose, the seat on which each cassette rests in a processing site includes guides cooperating with the base 12 of the cassette to ensure that the cassette is always positioned in the same manner on the seat. This allows linking the reference of the cassette to the reference of the robot.
The accuracy of the positioning depends upon the geometrical variations of each cassette. Often the geometry of the cassettes, molded in plastics, varies with time, so that some wafers will not always be in the position expected by the robot. The wafers are then usually broken by the robot.
A current geometrical variation of the cassettes is the warping of base 12, so that the cassette does not rest on the seat with a plane-to-plane contact. As a result the grooves are higher on one side than the other and the wafers are tilted instead of being horizontal.
A variation in height of the cassette causes a vertical shift of the grooves.
An inclination of the cassette with respect to its base causes a lateral shift of the grooves, the shift increasing with the elevational position of the grooves.
Various devices exist for controlling the dimensions of the cassettes.
Among the simplest control devices, Fluoroware's device, called "Wafer Carrier Go/No-Go Gauge," includes a drawer for sliding into the cassette to be controlled a set of fictive wafers in the form of a stack of disks. The cassette is of course suitably positioned with respect to a seat of the device. If resistance is felt to the penetration of the disks, it means that the cassette is bad.
This device does not enable an easy control of the planarity of the base of the cassette. Indeed, in this case, while the disks penetrate into the cassette, they tend to straighten the cassette without a high resistance being felt, this being due to the fact that the material of the cassettes has a low friction coefficient and that the walls of the grooves are little slanted.
Thus, the detection of a defect highly depends upon the sensitivity of the operator of the control device.
In addition, the device is particularly expensive, especially because it includes a stack of a large number of disks, often 25, which should be accurately manufactured and accurately mounted in the drawer.
Other devices allow more complete and accurate controls of cassettes, but they use measurement technologies which are sophisticated and significantly more expensive than those of the above described drawer device.